August 09, 2010

Local adaptation and admixture

This is a very interesting and well-written paper, and I highly recommend it (it's open access).

The authors deal with the problem of admixture between locally-adapted populations and newly introduced populations. Local adaptation is the occurrence of alleles and allele combinations that are well-suited to the local environment.

Admixture is a double-edged sword: on the one hand, it dilutes locally adapted gene pools by introducing foreign (non-adapted) alleles. On the other, it reduces homozygosity and inbreeding depression. Local adaptation is fitness enhancing, while inbreeding depression is fitness damaging. Whether one or the other wins out is probably case specific, and would depend e.g., on the level of adaptation, as well as the level of inbreeding depression in the population.

The paper does not address humans in particular, but a human example might be instructive. Consider the movement of tropically-adapted people into an arctic village. If inbreeding depression is substantial, then a proportion of the native-native (arctic) offspring would be homozygous for deleterious alleles and would not be able to compete successfully with invaders or native-invader offspring. On the other hand, invader and invader-native hybrids would lack locally adapted genomes (e.g. related to heat production) and would thus be at a disadvantage against functional natives.

The consequences of admixture in introduced populations are also interesting. Such populations lack local adaptation, and they are also often less genetically diverse (as they represent an often small subset of founders drawn from a larger non-native population). Thus, they benefit from admixture doubly: by receiving locally adapted variants, and by increasing their genetic diversity and thus becoming more capable of adaptation (*)

Thus, in considering the process of admixture between native and introduced populations, we must take into account a few factors:

The loss of local adaptation, generally reduced by admixture in the native population

The paper's utility is quite broad, and, while the authors do well not to limit themselves to a particular species, the implications for human societies are worth considering.

Proceedings of the Royal Society B doi:10.1098/rspb.2010.1272

Population admixture, biological invasions and the balance between local adaptation and inbreeding depression

Koen J. F. Verhoeven et al.

When previously isolated populations meet and mix, the resulting admixed population can benefit from several genetic advantages, including increased genetic variation, the creation of novel genotypes and the masking of deleterious mutations. These admixture benefits are thought to play an important role in biological invasions. In contrast, populations in their native range often remain differentiated and frequently suffer from inbreeding depression owing to isolation. While the advantages of admixture are evident for introduced populations that experienced recent bottlenecks or that face novel selection pressures, it is less obvious why native range populations do not similarly benefit from admixture. Here we argue that a temporary loss of local adaptation in recent invaders fundamentally alters the fitness consequences of admixture. In native populations, selection against dilution of the locally adapted gene pool inhibits unconstrained admixture and reinforces population isolation, with some level of inbreeding depression as an expected consequence. We show that admixture is selected against despite significant inbreeding depression because the benefits of local adaptation are greater than the cost of inbreeding. In contrast, introduced populations that have not yet established a pattern of local adaptation can freely reap the benefits of admixture. There can be strong selection for admixture because it instantly lifts the inbreeding depression that had built up in isolated parental populations. Recent work in Silene suggests that reduced inbreeding depression associated with post-introduction admixture may contribute to enhanced fitness of invasive populations. We hypothesize that in locally adapted populations, the benefits of local adaptation are balanced against an inbreeding cost that could develop in part owing to the isolating effect of local adaptation itself. The inbreeding cost can be revealed in admixing populations during recent invasions.

7 comments:

In the human case, the fitness of invaders has little to do with the genetic package they bring, compared with the cultural package that they deliver.

For example, in Greenland, the Nordic arrivals 1000 CE were replaced by the locals, despite the fact that both populations had genetic adaptations to cold weather living, because the Nordic arrivals tried to farm and herd at a time ill suited for doing that, and the locals didn't. Historical human population replacements appear to have been frequently driven by cultural packages and not genetics.

Moreover, the culture package that the newcomers bring with them changes what genetic traits will be more fit. Lactose tolerance, for example, will become more valuable if the newcomers transfer herding as part of their cultural package.

The optimal scenario in most human cases, it would seem, would be to have critical masses of both populations and a great deal of admixture, so that selection could favor the genetic traits in the old and the new populations in the culturally transformed environment that results from the fusion of the two. When cows arrive in Sweden from Southern Europe, the ideally adapted population would have both lactose tolerance and cold weather/high latitude adaptations, drawing some genes from one population and some from the other, even though some of the new genes weren't adaptive before the populations fused.

Indeed, it is hard to imagine a human population replacement taking hold without some advantageous cultural package. Historically, the indigenous populations that have best resisted being overwhelmed by invaders are those who have quickly adopted the advantageous cultural innovations of their invaders.

So, this is an area where the conclusion from animal models is likely to be a poor fit for human experience.

"In native populations, selection against dilution of the locally adapted gene pool inhibits unconstrained admixture and reinforces population isolation, with some level of inbreeding depression as an expected consequence".

To my way of thinking that is how haplogroups have been able to move into new populations without altering the existing local overall phenotype particularly much.

"Recent work in Silene suggests that reduced inbreeding depression associated with post-introduction admixture may contribute to enhanced fitness of invasive populations".

And then, in turn, the population with 'enhanced fitness' can expand further. I strongly suspect that is the origin of such expansions as the Austronesian and Indo-European, and presumably others. Perhaps the Acheulean and Levallois expansions.

"The paper's utility is quite broad, and, while the authors do well not to limit themselves to a particular species, the implications for human societies are worth considering".

Not to mention implications for the mechanism of evolution as a whole. I'm fairly sure that occasions when a small inbred population expands as a new species are few and far between. Far more likely is the apparent gradual change and diversification of species through the interaction of inbreeding and hybrid vigour studied in this article. For any interested that's exactly what I claimed in this old essay:

"So, this is an area where the conclusion from animal models is likely to be a poor fit for human experience".

But we can fit it to the human experience if we keep in mind your previous comments:

"Moreover, the culture package that the newcomers bring with them changes what genetic traits will be more fit".

And:

"The optimal scenario in most human cases, it would seem, would be to have critical masses of both populations and a great deal of admixture, so that selection could favor the genetic traits in the old and the new populations in the culturally transformed environment that results from the fusion of the two".

Although I believe we don't need 'critical masses of both populations'. Two small inbred populations would be sufficient.

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